This application claims the priority of German Patent Application, Serial No. 102 00 680.6, filed Jan. 10, 2002, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.
The present invention relates to a method for a jerk-limited speed control of a movable machine element of a numerically controlled industrial processing machine, such as a machine tool, a robot and the like, wherein a travel path of the machine element is subdivided into consecutive travel segments, as well as a corresponding numerical controller.
A mechanical system capable of oscillating (e.g., axes of machine tools) typically has at least one characteristic frequency which is excited during travel and is reflected in the actual position value. An excitation of the mechanism should therefore be prevented or at least reduced.
Jerk limitation is nowadays customarily implemented in speed controls of a numerical controller, e.g., a controller used for a machine tool or a robot. The goal is to reduce loading of the individual axes of the machine without detrimentally affecting the program processing time.
Jerk limitation can delay the buildup of an acceleration in the travel so as to smooth the set point and move the mechanism to minimize the excitation of oscillations.
This smoothing effect of a jerk limitation depends strongly of the desired curve for the set point. Measurements and theoretical investigations have shown that the dynamic range of higher frequency acceleration and braking processes has to be reduced, so that the oscillatable mechanism is not excited too strongly. This dependence holds both for short positioning processes as well as for controlling the entire travel path.
In conventional systems, this problem is addressed by setting small jerk and acceleration limit values, so that high-frequency changes in the path velocity do not strongly excite the oscillations. Disadvantageously, the small preset dynamic values frequently prevent a higher path velocity and hence a shorter program processing time.
A conventional method for attaining a velocity profile with reduced wear on the machine utilizes jerk limitation. For example, in phase 1, the acceleration is set to a maximum value. In phase 2, a constant acceleration is maintained so that the velocity increases linearly. In phase 3, the acceleration is decreased. The term acceleration herein also includes a negative acceleration (braking).
At the end of phase 3, the path velocity reaches a maximum, wherein this velocity is maintained in phase 4. In additional phases 5 to 7, the velocity is decreased in an analogous fashion, so that the path velocity at a set position becomes zero. For this purpose, a negative acceleration is built up in phase 5, which is kept constant in phase 6 and then decreases to zero in phase 7. The slope of the acceleration in phases 1, 3, 5 and 7 is critical for the jerk of a machine. The velocity curve in these phases can be described by a polynomial, whereas in the other phases the velocity is a linear function of time or constant.
So far, the time dependence of the jerk r(t) can be represented by constant basic shapes in the form of rectangles. FIG. 1 depicts the time dependence of a jerk as a function of the times tr, ta0 and tv0, which are the time intervals corresponding to the jerk r, the phase with a constant acceleration a, and the travel with a constant velocity v. The jerk r(t) is either equal to zero or equal to a maximum allowable value r0 which can have a positive or negative sign: r(t)r(t)xcex5{0,+r0,xe2x88x92r0}.
This type of jerk limitation, depending on the way it is implemented, typically prolongs the program processing time in comparison to a velocity control where the acceleration is limited. A shorter processing time may be achievable if a larger acceleration value can be set for the machine within the jerk limit values.
As mentioned above, however, such highly dynamic acceleration and braking processes along the path can disadvantageously excite mechanical oscillations in certain frequency ranges. The dynamics of such processes should therefore be adapted to the machine characteristics.
It would therefore be desirable and advantageous to provide an improved method and system that limit jerk so that critical resonance frequencies of a machine or a movable machine element are not excited.
According to one aspect of the present invention, a method for a jerk-limited speed control of a movable machine element of a numerically controlled industrial processing machine, such as a machine tool, a robot and the like, includes subdividing a travel path of the machine element into consecutive travel segments, determining a jerk profile for each of the travel segments, and adapting the jerk profiles by a jerk limitation provided by a filtering action through bandstop filters having blocking frequencies, wherein the blocking frequencies coincide substantially with at least a selected number of characteristic frequencies of the movable machine element.
According to another aspect of the invention, a numerical controller for a jerk-limited speed control of a movable machine element of a numerically controlled industrial processing machine, such as a machine tool, a robot and the like, includes an interpolator for subdividing a travel path of the machine element into consecutive travel segments. The controller further includes a filter having a filtering function in form of a bandstop filter, with the bandstop filter adapting jerk profiles by a jerk limitation so that blocking frequencies of the bandstop filter coincide substantially with at least a selected number of characteristic frequencies of the movable machine element.
According to an advantageous feature of the present invention, at least one of the parameters jerk, acceleration and velocity is preset so as to adapt the jerk profile to the blocking frequencies. The jerk profile can include a rectangular jerk curve, wherein different constant jerk values are defined for the rectangular jerk curve for adapting the frequencies of the bandstop filter. In still another embodiment, for adapting the frequencies of the bandstop filter, the shape of the jerk profile is changed by predefining a corresponding shape function for a jerk value of the jerk profile.
According to another advantageous feature of the present invention, the jerk profile can changed by rounding a rectangular jerk curve. This can be done effectively by rounding the rectangular jerk profile with a harmonic function, in particular the square of a sine function (sin2).
The length of the travel path can remain unchanged by changing the shape of the jerk profile for the same jerk duration so that the area of the jerk profile after the change in the shape of the jerk profile corresponds to the area of the jerk profile before the change of the shape. For the case of a rectangular jerk profile and for the same jerk duration, the maximum jerk value of the jerk profile having the changed shape is increased until the area of the jerk profile is equal to the area of the rectangular jerk curve.
According to another advantageous feature of the present invention, the jerk profiles are determined in advance over several sets of the travel segments on the travel path of the machine element for adapting the jerk profiles.
According to yet another advantageous feature of the present invention, the machine element can have a plurality of axes, wherein the jerk profile can be adapted separately for each axis subject to a jerk by analyzing a corresponding axial velocity curve based on an assumed constant path velocity of the machine element. Advantageously, axial jerk profiles can be determined in advance over several sets of the travel segments on the travel path of the machine element, wherein each segment is defined by a minimum or a maximum on the axial velocity curve.
According to yet another advantageous feature of the present invention, the jerk limitation may be implemented as a digital filter which supplies at each operating point of the machine element an associated controlled jerk value.
The aforedescribed intended adaptation of the jerk curve prevents excitation of resonances. The dynamic limit values can hence also be increased, thereby improving the program processing time.
The aforedescribed jerk limitation according to the invention results, i.a., in the following advantages.
Substantially reduced excitation of oscillations during travel over the same path in the same time. As a result, machine tools travel more softly and more accurately during surface machining and profile cutting, and the quality is also improved.
Faster travel for the same oscillation excitation with a greater permissible jerk. For machine tools, this speeds up machining while maintaining the same quality, increases the output of manufactured parts over the same time period, and improves efficiency.